PLANT ECOLOGY 313 



for food storage, and such shoots often become modified as 

 food depositories, as in bulbs, tubers, rootstocks, etc. In 

 the scaly buds the structure is used for protection rather 

 than storage. 



The stem bearing floral leaves is the shoot ordinarily 

 called "the flower," whose structure and work have been 

 sufficiently described. Its adjustments have in view polli- 

 nation and seed dispersal, two very great ecological sub- 

 jects full of interesting details. 



176. Roots. — Roots are absorbent organs or holdfasts or 

 both, and they enter into a variety of relations. Most 

 common is the soil relation, and the energetic way in 

 which such roots penetrate the soil, and search in every 

 direction for water and absorb it, proves them to be highly 

 organized members. Then there are roots related to free 

 water, and others to air, each with its appropriate struc- 

 ture. More mechanical are the clinging roots (ivies, etc.), 

 and prop roots (screw pines, banyans, etc.), but their adap- 

 tation to the peculiar service they render is none the less 

 interesting. 



The above statements concerning leaves, shoots, and 

 roots should be applied with necessary modifications to the 

 lower plants which do not produce such organs. The 

 light relation and its demands are no less real among the 

 Algae than among Spermatophytes, as well as relations to 

 air, soil, water, mechanical support, etc. 



PLANT ASSOCIATIONS 



177. Introductory.— Plants are not scattered at hap- 

 hazard over the surface of the earth, but are organized into 

 definite communities. These communities are determined 

 by the conditions of living — conditions which admit some 

 plants and forbid others. Such an assemblage of plants 

 living together in similar conditions is a plant association. 

 Closely related plants are the most intense rivals, as they 



